Gaseous waste products resulting from the combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbons and nitrogen oxides as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. It is well known to use catalytic composites to simultaneously convert carbon monoxide, hydrocarbon and nitrogen oxide pollutants to innocuous gases. In order to achieve the simultaneous conversion of said pollutants, a catalytic composite (commonly called a three component control catalyst) is ordinarily used in conjunction with an air/fuel ratio control means which functions in response to a feedback signal from an oxygen sensor in the engine exhaust systems. The air-to-fuel ratio control means is typically programmed to provide fuel and air to the engine at a ratio at or near the stoichiometric balance of oxidants and reductants in the hot exhaust gases at engine cruising conditions, and to a stoichiometric excess of reductants during engine startup and at engine acceleration conditions. The result is that the composition of the exhaust gases with which the catalyst is contacted fluctuates almost constantly, such that conditions to which the catalyst is exposed are alternatively net-reducing (fuel rich) and net-oxidizing (fuel lean). A catalyst for the oxidation of carbon monoxide and hydrocarbons and the reduction of nitric oxide must be capable of operating in such a dynamic environment.
The exhaust gas also contains other components such as sulfur oxides, phosphorus and zinc compounds which are known catalyst poisons. The sulfur oxides present in the exhaust stream can react with the catalyst to form other products. For example under fuel lean (net-oxidizing) conditions, sulfur dioxide (SO.sub.2) reacts with oxygen (O.sub.2) over the catalyst to form sulfur trioxide (SO.sub.3) which is then converted to sulfates (SO.sub.4.sup.=) by reaction with water. Under fuel rich (net-reducing) conditions the SO.sub.2 reacts with hydrogen (H.sub.2) to form hydrogen sulfide (H.sub.2 S). The formation of H.sub.2 S is particularly objectionable because of its strong odor.
In addition to the formation of H.sub.2 S over a noble metal catalyst, a storage phenomenon has also been observed. This storage phenomenon has been documented in the literature, G. J. Barnes and J. C. Summers, "Hydrogen Sulfide Formation Over Automotive Oxidation Catalysts," Society of Automotive Engineers, Paper No. 750093. The experimenters showed that sulfur accumulated on noble metal catalysts under both oxidizing and reducing atmospheres. For example, under oxidizing conditions the sulfur is typically stored as sulfates (SO.sub.4.sup.=) which is converted to H.sub.2 S under reducing conditions.
Although this phenomenon has been recognized for many years, the problem which it generates, i.e. unpleasant odor, was relatively minor and was not of much concern until recently. In the past few years automotive catalyst technology has improved so that the catalysts are much more active than previous catalysts. Part of this improvement has been achieved by increasing the content of the oxygen storage component present in the catalytic composite. The most commonly used oxygen storage components are the rare earths. Unfortunately, the rare earths appear to increase the storage of sulfur during fuel lean operation, and when release occurs the concentration of hydrogen sulfide is much larger than would have been anticipated, based on the sulfur content of the fuel. Consequently, the resultant odor is quite noticeable and many more drivers are offended by the increased hydrogen sulfide odor.
Since the odor has become more noticeable and objectionable, a need exists to minimize the hydrogen sulfide emissions from catalyst equipped automobiles. The instant invention cures this problem by providing a catalytic composite in which the oxygen storage component is separated from the noble metal component. This is accomplished by depositing the noble metal component on a first support which is a refractory inorganic oxide and then depositing a layer consisting of an oxygen storage component and optionally a second support which is a refractory inorganic oxide immediately thereover.
The prior art, U.S. Pat. No. 3,873,469, does disclose a layered catalytic composite. Additionally, Japanese Public Disclosures 71537/87 and 71538/87 disclose a catalytic composite consisting of a ceramic carrier having dispersed thereon a catalytic layer containing one or more of Pd, Pt and Rh and an alumina layer containing one or more oxides of Ce, Ni, Mo and Fe. However, the stated advantage of the 71537 invention is that the oxides, which are oxygen storage components, renew the catalytic surface. This is accomplished by having the oxygen storage component in contact with the catalytic surface.
In contrast to this prior art, the present invention separates the catalytic layer from the oxygen storage component. This separation is in contravention to the prior art which states that intimate contact of the oxygen storage component with the catalytic metal is required in order for the oxygen storage component to be effective. Therefore, the instant invention differs from the prior art in that the oxygen storage component is separated from the catalytic or noble metal component. Additionally, the result of this difference is to minimize the formation of H.sub.2 S over a catalytic composite, even though said catalytic composite contains the same or greater amount of an oxygen storage component as a conventional catalyst, a result quite distinct from that which could reasonably be expected from the teachings of the prior art.